1. Field of the Invention
The present invention relates to a heat pump air conditioning system performimg a refrigeration cycle having a compressor for compressing refrigerant, means for exchanging heat including an evaporator for evaporating the refrigerant and a condenser for condensing the refrigerant, and an expansion valve for controlling the passage of the refrigerant and more particularly to an improvement of coefficients of performance by an active control of a refrigeration cycle in the system.
2. Description of the Prior Art
Recently, keen attention has been paid to a heat pump air conditioning system for the purpose of improving energy efficiency of the air conditioning system. However, in a case, where a system is used both for a heating and cooling mode by reversing the flow direction of refrigerant back and forth, it is difficult to optimize both its coefficients of performance for the heating and cooling modes of operation, since both a temperature difference between indoor and outdoor environments and an indoor heat load vary in most cases between the two modes.
In the prior art, efforts have been made to improve the performance of both the heating and cooling modes of the heat pump air conditioning system, e.g. in U.S. Ser. No. 465,798 by Reedy et. al., by changing the placement and sizes of an expansion valve between the two modes.
In such a prior art system, however, the coefficients of performance generally tend to be lowered when the outdoor temperature becomes either too low or too high compared to a nominal design temperature, even if the coefficient of performance is maximized or optimized for the nominal indoor and outdoor temperatures and the indoor heat load at the nominal design temperature, since the opening of an expansion valve and rotational speed of a rotary compressor are almost fixed at this design condition in the prior art heat pump air conditioning system.
Recently, it has been proposed as an energy efficient air conditioning system e.g., in U.S. Pat. No. 4,246,956 to Drucker, that solar collectors be equipped in parallel with the heat pump air conditioning system. This system has tried to utilize solar radiation heat to assist the heat collection capability of an existing heat pump air conditioning system. It is, however, very difficult to continuously obtain the maximum coefficient of performance with regard to variable heat input such as solar radiation, since an opening of the expansion valve and a rotation speed of the rotary compressor are fixed and thus cannot adjust themselves to the varying environment in the above heat pump air conditioning system.
In a prior art central air conditioning system, and in particular, in a central heating system, the room temperature of a house is regulated collectively, not individually, based upon the average temperature thereof. This system has such disadvantages that unused rooms are warmed up unnecessarily, resulting in energy waste, while rooms remote from the heat source tend to have a lower temperature due to the temperature drop in the pipings, etc.